(1) Field of the Invention
The present invention relates to a method for the chemical treatment of woods in which a treating solution of a warp-preventing chemical, a hardening agent, an insecticide or a wood preservative is heated at temperatures not causing high temperature troubles such as discoloration in wood and the heated treating solution is intruded promptly and uniformly into the wood under a pressure higher than the saturated vapor pressure.
(2) Description of the Prior Art
It is known that woods such as beech, oak, larch, pine, hemlock, lauan, kempas, red sandalwood, ebony, rosewood, apitong, kapor, monkeypot, walnut, maple and gum tree have been subjected to permeation treatments with aqueous solutions of surface active agents and dilute alkali solutions.
According to this known method, an aqueous solution of a surface active agent is charged in a boiling kettle and wood is dipped in this solution and boiled for 4 to 12 hours (the first step). The wood is then taken out from the kettle and transferred into a pressure vessel. A treating solution comprising an aqueous solution of a surface active agent and a dilute alkali solution is introduced into the pressure vessel and heat treatment is carried out under pressure for 8 to 9 hours while maintaining the treating solution at 120.degree. to 130.degree. C. and 2 to 2.75 Kgf/cm.sup.2.a (1 to 2 Kgf/cm.sup.2.a) with steam (the second step). Accordingly, in this known method, a very long time is required for completion of the treatment and much labor is necessary for transferring wood from the boiling kettle to the pressure vessel.
This defect is due to the fact that it had been considered that steam is the only means for permeation of the treating solution under heat and pressure.
More specifically, since woods have ordinarily poor heat resistance and the uppermost temperature not causing high temperature troubles such as discoloration is about 130.degree. C., if pressurization is effected with steam while maintaining the temperature of the treating solution at a level not exceeding 130.degree. C., it is impossible to elevate the pressure above about 2.75 Kgf/cm.sup.2.a, which is the saturated steam pressure at 130.degree. C.
If the temperature is controlled within the range not causing high temperature troubles, the heat resistance of woods is satisfactory, but the pressure cannot be elevated above the low level of about 2.75 Kgf/cm.sup.2.a. Therefore, a considerably long time is required for the treating solution to permeate into the interior of wood.
When wood is charged in the pressure vessel from the start of the treatment and the treating solution is heated and pressurized by steam, not only the treating solution but also the wood per se will be heated. Accordingly, in this case, the heating time is extremely prolonged. Therefore, in the conventional method, in order to shorten the heating time in the pressure vessel, the boiling treatment which hardly attains any permeating effect is carried out at the first step so as to elevate the temperature of the wood.
In the conventional method, even if a large quantity of steam is supplied to the pressure vessel, it is impossible to elevate the pressure of the treating solution above 1 atmosphere while the temperature is still lower than 100.degree. C., and in this case, the steaming step is nothing but a heating step and it is impossible to cause the treating solution to permeate under pressure into the wood. This is another fatal defect of the conventional method.
When the temperature of the treating solution is elevated to a predetermined level by steam supplied into the pressure vessel, even if the surface portion of wood is heated at a temperature substantially equal to the temperature of the treating solution, the temperature of the interior portion of wood is still low. Accordingly, the temperature of the treating solution is lowered and supplied steam is steadily liquefied. Furthermore, even while the solution temperature and pressure are maintained at predetermined levels, steam is liquefied. Accordingly, the concentration of the treating solution is drastically reduced and the intended chemical treating effect cannot be attained satisfactorily. This is still another defect of the conventional method.